Reference is made to my own earlier U.S. Pat. No. 4,474,335, dated Oct. 2, 1984, and entitled "Apparatus for Centrifugal Pulpwood and Woodchip Grinding". This patent basically discloses an apparatus operating as described in the previous section, with the addition of means incorporated in the rotor for applying water to the grinding surface, thus utilizing the centrifugal force to increase the water pressure at a spraying location remote from the axis of rotation.
It will be appreciated that the apparatus set forth in the aforesaid U.S. Pat. No. 4,474,335 represents a departure from conventional grinding procedures, in which the wood is urged against the outer periphery of a typically cylindrical grinding stone. In connection with this conventional procedure, it is known to grind the wood under superatmospheric pressure, thus permitting grinding temperatures higher than in the standard stone groundwood (SGW). In U.S. Pat. Nos. 3,808,090 and 3,948,449, a process is described for improving the groundwood pulp by grinding wood in a closed grinding chamber in a pressurized gaseous atmosphere. In the two patents just named, the wood is fed in and the superatmospheric pressure in the grinding chamber can be maintained only so long as the grinding of a wood batch continues. However, when a new wood batch must be fed into the magazine, the magazine must be opened and the pressure of the grinding surface falls to atmospheric. Thus, the grinder of these two patents does not work in a continuously pressurized atmosphere.
Further developments in this technology are exemplified in Canadian Pat. No. 1,097,118, issued Mar. 10, 1981, and U.S. Pat. Nos. 4,270,703 and 4,274,600, issued June 2, 1981 and June 23, 1981, respectively. These patents are assigned to Oy Tampella Ab. In the Oy Tampella process, a feed chamber upstream of the grinding chamber has two pressure seals, one to the atmosphere and one to the grinding chamber. Thus, the feed chamber acts as a double-lock seal, to allow the pressure in the grinding chamber always to be maintained above atmospheric. By the use of this method, the pressure in the chamber may reach as high as several bar, and temperatures at the grinding stone surface may climb well above the standard pressure boiling point.
The apparatus set forth in my earlier U.S. Pat. No. 4,474,335, identified above, presents a desirable alternative to the SGW process and the pressurized groundwood (PGW) process, in that the centrifugal effect generated by the rotor can be utilized not only to press the pulpwood or woodchips against the grinding surface, but also to create pressurized effects without the need for a pressure lock mechanism. In other words, because the material is being "flung" against the internal grinding surface, the water in the slurry acts as if it were pressurized above atmospheric, and can attain temperatures above the normal boiling point of water without boiling.
It is now appropriate to discuss a further aspect that has been encountered in connection with the centrifugal grinding apparatus disclosed in my earlier U.S. Pat. No. 4,474,335. While the apparatus described in that patent functions quite satisfactorily, it has been concluded that for centrifugal grinders of acceptable size, the necessary rotational rate to achieve the required grinding force through the centrifugal effect may result in a rubbing speed which is faster than that which produces optimum pulp quality. More specifically, for a centrifugal grinder of acceptable dimensions, rubbing speeds on the order of 10,000 to 12,000 feet per minute may be required in order to achieve the necessary grinding force through the centrifugal effect. However, practical experience gained from conventional grinders suggests that optimum pulp quality occurs at speeds of less than 6,000 feet per minute.
It has been concluded that the solution to this problem does not lie in slowing down the rotor to achieve a rubbing speed of 6,000 feet per minute or less, since the centrifugal grinding force varies as the square of the rotational speed, and would drop to one quarter of that achieved at a rubbing speed of 12,000 feet per minute.
It can thus be seen from the above discussion that a desirable development in connection with centrifugal grinders would be one which provided an extra pressure factor urging woodchips against the internal grinding surface, so that it would not be necessary to rotate the rotor so fast that the rubbing speed produced an unacceptable pulp quality.
To complete the summary of the prior art, reference may be had to U.S. Pat. No. 4,456,503 issued June 26, 1984 to Bystedt, which discloses the use of a pressurized grinding chamber primarily for the purpose of raising the temperature to between 110.degree. and 130.degree. C., at which temperature the lignin in the wood bond softens. The patent describes a steam-tight plug of fibers which is intended to contain the pressure of steam ahead of the plug, in order that the general temperature of the woodchip grinding region can be raised above the boiling point at atmospheric pressure.
Other U.S. Pat. Nos. of interest are the following: U.S. Pat. No. 4,017,356, issued Apr. 12, 1977 to Bystedt, U.S. Pat. No. 4,445,973, issued May 1, 1984 to Bystedt, U.S. Pat. No. 4,034,870, issued July 12, 1977 to Duch, U.S. Pat. No. 3,687,749, issued Aug. 29, 1972 to Reinhall.